Daphnia exhibit diurnal vertical migration in shallow rock-pools

Esa Ranta & Visa Nuutinen Department of Zoology University of Helsinki P. Rautatiekatu 13 SF-00100 Helsinki Finland

Keywords: Vertical migration, Daphnia, shallow rock-pools

Abstract

We report diurnal changes in vertical distribution of Daphnia longispina and D. magna in shallow (25-60 cm) rock-pools on Baltic islands, southern Finland. During the daytime shallow rock-pool Daphnia reside in water layers close to the sediment while at midnight they either are evenly distributed throughout the water column, or have a modal peak of distribution in the surface water. Contrary to this pattern, no changes were observed in vertical distrjbution of D. longispina at noon and at midnight in a 150 cm deep pond. We suggest that the pattern of Daphina vertical migration in shallow rock-pools is not satisfactorily explained by predation.

Introduction

The various species of zooplankton are often confined to relatively restricted zones and therefore exhibit uneven distribution in the water column. Diurnal changes in zooplankton vertical distribu- tion have long puzzled ecologists (Hutchinson, 1967: 725-809). In most of the documented cases, zooplankton vertical migration is associated with deep (even down to tens of metres) thermally strati- fied lakes (Hutchinson, 1967). In this paper we report diurnal changes in vertical distribution of Daphnia in shallow rock-pools. Rock-pools are small but numerous fresh and brackish water basins on the islands of the northern Baltic Sea (Levander, 1900). Large Daphnia ( D . magna Straus, D. lon- gispina 0. F. Miiller, D. pulex DeGeer) are com- mon and abundant zooplankton of rock-pools (Ranta, 1979).

Material and methods

Data were collected at noon (1 1.20- 12.00) and at

midnight (23.15-0.35) on 2-4 August, 1984. Al- together 9 rock-pools were sampled from 4 islands in the Tvarminne archipelago, southern Finland. Each pool was sampled twice, at noon and at mid- night. The rock-pools studied vary in size, in Daph- nia species present (only single-species pools were studied) and in resident planktivores (Table 1). A plankton net (diameter 10.5 cm, 65 pm mesh) at- tached to a 2.5 m rod was towed horizontally both in th'e surface water and as close to the sediment as possible, with 1 to 5 replicated 1 metre long tows constituting a sample. Care was taken to prevent mixing of the water by sampling. Daphnia were preserved in 70% ethanol and counted under a dis- secting microscope (l2X). In counting D. longispi- nu, we distinguished gravid and non-gravid fe- males.

We examined whether there were any significant differences in vertical distribution of Daphnia in the noon and midnight samples, as well as whether the gravid D. longispina females displayed different vertical distribution as compared to non-gravid Daphnia.

Hydrobiologia 127, 253-256 (1985). O Dr W. Junk Publishers, Dordrecht. Printed in the Netherlands.

Table I . Characterization of the 9 rock-pools studied. The deepest sampling depth refers also to the depth of the rock-pool. Number of replicated samples is 5 with the following exceptions: one in Storsundsharund C and D, four in Brannskar B26c. Day and night samples are treated separately, in D. longispina numbers of egg-carrying females are given first, then those of non-gravid and juvenile Daphnia.

Secchi disk Sampling depths Sample Important reading Water temperature size predators

D. longispina populations Lhgskar Tench Pond 20 cm 80 cm 150 cm

Day 90 cm 23°C 2OoC 18OC 82 2587 Tinca tinca Night 30 cm 20°C 19°C 18OC 277 3221 Chaoborus

Lhgskar Newt Pond

Brannskar 65c

Brlnnskar 26c

D. magna populations LBngskar Storgrunden

Storsundsharund A

Storsundsharund B

Storsundsharund C

Storsundsharund D

Day 49 cm Night 17 cm

Day 30 cm Night 12 cm

Day 20 cm Night 10 cm

Day 50 cm Night 50 cm

Day 40 cm Night 40 cm

Day 20 cm Night 10 cm

Day 20 cm Night 15 crn

Day 20 cm Night 20 cm

Results and discussion

Invariably, at noon in the shallow rock-pools Daphnia reside in deep waters while at midnight they are either evenly distributed or concentrated at the surface (Fig. 1). The vertical distribution of gravid females in two of the three shallow rock- pools with D. longispina populations differs signif- icantly from the distribution of non-gravid individ- uals (Fig. 1). In daytime gravid females reside more in deep water while at midnight their vertical distri- bution is more even in the water column. Inter- estingly enough, in the LBngskar Tench Pond (maximum depth 150 cm) we could not observe sig-

Triturus vulgaris Notonecta, Lesres Polyphemus

Corixidae Polyphemus

Corixidae

Corixidae Gammarus

Corixidae Gammarus

Corixidae

Corixidae

Corixidae

nificant differences in vertical distribution of D. longispina between noon and midnight samples.

Predation and vertical migration of Daphnia in rock-pools

Recently zooplankton vertical migration has been closely connected with size selective plankti- vory (Hall et al., 1976; Zaret & Suffern, 1976; Zaret, 1980; Wright et al., 1980; Vuorinen et al., 1983; but see also McLaren, 1974 and Enright, 1977). The predation hypothesis states that large plankters avoid visually hunting predators by migrating down to deep and turbid waters during daytime. At

Fig. I . Daphnia population densities (ind, I 1; In x+l trans- formed data, mean f S.D. ) in surface and bottom water of 9 rock-pools in the Tvarminne archipelago. Noon(open bars) and midnight (shaded bars) samples are separated. For D. longispina populations the vertical distribution of egg-carrying females is indicated with inserted percentages. In all cases noon and mid- night vertical distributions of Daphnia in shallow rock-pools differ significantly (G-tests, P<< 0.001). with the exception of the Llngskar Tench Pond (LTP). Also, in the shallow rock- pools, with the exception of Brannskar B26c, gravid D. longis- pina females reside in different depths at noon and at midnigth (G-tests, P<0.01). Rock-pools studied are: LNP= Llngskar Newt Pond, B26c= Brannskar 26c, B65c= Brannskar65c, LTP= Llngskar Tench Pond, LSG= Llnskar Storgrunden, SA-SD= Storsundsharund A-D.

dusk, with low illumination, the risk of being eaten diminishes in surface waters and the zooplankton rise to graze on phytoplankton.

Visually hunting planktivores in rock-pools are usually corixids and diving beetles (Lindberg, 1944) although some of the largest pools and ponds have fish (tench (Tinca tinca L.), sticklebacks (Pungitius pungitius L. and Gasterosteus aculeatus L.)) or smooth newt (Triturus vulgaris L.) populations. Elsewhere (Ranta & Nuutinen, 1984, 1985) it has been shown that smooth newts, tench and stickle- backs have a preference for large zooplankton prey. Thus, in fish or newt inhabited pools the expecta-

tion is for predation related vertical migration of Daphnia. In the Llngskar Tench Pond no migra- tion by D. longispina was observed (though our data show that tench readily feed on Daphnia (Ran- ta & Nuutinen, 1984)); yet in the LBngskar Newt Pond a clear diurnal migration was seen (Fig. 1).

Two species of corixids (Arctocorisa carinata Sahlberg, Callicorixa producta Reuter) are the predominant potential rockpool invertebrate planktivores. All developmental stages of these species show a preference for small over large D. magna (Ranta & Espo, in prep.). However, when benthic chironomids are available as alternative food, the corixids shift to forage almost exclusively on them. In rock-pools chironomid larvae are al- ways present (Lindberg, 1944) though their abun- dances vary.

In shallow rock-pools Daphnia that reside close to the bottom sediment are vulnerable to corixid predation because during daytime, corixids prey on chironomid larvae in the sediment. If no chironom- ids are available, Daphnia close to the sediment surface are easy prey to cotixids since no microhab- itat shift is needed.

We find the data on Daphnia vertical migration in shallow rock-pools difficult to explain in terms of predation. In these pools secchi disc readings at noon are to the bottom. Thus, Daphnia have no low-light refuges in shallow rock-pools yet during the day they stay in deep waters. Problems also arise with the D. longispina population in the Lbngskar Tench Pond where we found no evidence of diurnal migration. This finding contrasts with the other D. longispina populations studied which do exhibit a diurnal migration pattern.

Weider (1984) recently documented that in a 6 m deep fishless Illinois pond, the D. pulex population consisted of distinct genotypes, some of which dis- played vertical migration and some did not. Whether this is valid for the migrating and non- migrating D. longispina populations in Tvarminne area is not known to us.

Finding an explanation for zooplankton vertical migration has been the subject of numerous studies (see Hutchinson, 1967: 755-779 for experiments prior to the predation hypothesis and Zaret, 1980 for predation motivated experiments). It is interest- ing to note that the daytime thermal gradient in shallow rock-pools is isothermal at night (Table 1). For example, Kikuchi (1938) demonstrated that an

increase of water temperature from 14 O C to c. 20 " C changed the distribution of Daphnia from even to uneven with a modal peak at the bottom of the experimental tube.

We conclude that there are both field and exper- imental data which suggest that putting too much faith in predation (for alternatives, see Hutchinson, 1967; Segal, 1970) as the single explanation for all types of zooplankton diurnal vertical migration stretches one's credulity.

Acknowledgements

We thank J. Sarvala and I. Vuorinen and S. F. Tjossem for their comments on the manuscript.

References

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Received 15 October 1984; in revised form 20 April 1985; accepted 28 April 1985.